Personally, if I had to make a guess, I’d say the humans, the dwarves ( all three clans united), gnomes and mechagnomes united, the Zandalari after they bring the other troll tribes under their rule, goblins, and the Forsaken are the best canditates for winning a war.
I think this because all of these sides still seem to have relatively enough numbers to wage war, aswell as having enough of an edge where it comes to either magical abilities or technology.
Humans are a versatile race with their human potential meme, and pretty much spread all over the globe, and have both magic and technology to hold against the others. That and they have the worgen ( who for convenience sake I just pair up with the humans).
The dwarven clans make up for each other’s weaknesses quite well imo, where the Bronzebeards bring most of the standard, solid military forces. The Wildhammers bring potential air superiority to the table aswell as aid from the elements through their shaman. And the Dark Irons finally bring magic, the knowledge to make golems, and the willingness to fight dirty. And all three clans together can still form quite the numbers I think.
Gnomes and Mechagnomes are smaller in number, but they have the technology to gain the upper hand.
Same goes for goblins, and depending on if it’s just the Bilgewater Cartel we are talking about here, or all the cartels uniting, and they are definetly more numerous than the mechagnomes and gnomes.
The Zandalari, even without the other troll tribes, are also quite numerous, they have their dinosaurs, and not to mention a boatload of Loa to ask for intervention should it be necessary.
The Forsaken finally, depends on how they will fight during the post-Sylvanas age. If they still cling to the use of the plague and other forms of chemical warfare, aswell as continually raising their foes into their ranks, I think they too will be solid contenders for world domination. Also don’t forget they need no food, sleep or other sustenance and can essentially fight anywhere, anytime.
My estimates are made on actually going on the offensive and not just turtling in their own respective homes, aswell as numbers.